Hydrogen as Fuel

Diagram of a hydrogen fuel cell based on a polymer electrolyte membrane. An anode attracts anions (–), whereas a cathode attracts cations (+). Original drawing courtesy of Jafet.

Internal combustion engines with some modifications can burn hydrogen, but the thermodynamics of such engines limit their efficiency to about 23%, somewhat independently of the fuel being combusted. This means that less than a quarter of the energy in the fuel is converted into propelling the vehicle.

Fuel cells, in which hydrogen reacts with oxygen to form water and generates electricity for an electric motor that moves a vehicle, may be more efficient, theoretically above 36%. [1], [2] This approach, however, depends on advances in hydrogen storage, fuel cell costs, fuel cell reliability, and hydrogen production and distribution.

Vehicles running on alternative fuels need a range of travel comparable to that of gasoline vehicles, something greater than 300 km (186 miles), before they will be adopted for general use. Unfortunately, hydrogen has the lowest energy content per unit volume of any fuel. To carry enough hydrogen for an adequate travel range, a vehicle must accommodate either compressed hydrogen gas, liquefied hydrogen gas, or hydrogen gas absorbed onto solid particles.

To match energy contents with natural gas, hydrogen must be compressed to a much higher pressure: A tank filled with 10,000 psi of hydrogen (680 times the normal atmospheric pressure) holds the same amount of energy as one filled with 2000 psi of CNG (compressed natural gas). Under such pressures, hydrogen is such a small, energetic molecule that it diffuses through most materials that line the walls of compressed gas tanks. Once hydrogen penetrates the liners, it reacts with many wall materials (e.g., steel, aluminum, and titanium), and weakens them, a process called embrittlement. Lightweight storage tanks that can withstand hydrogen at 10,000 psi have liners of high molecular weight polymers, walls of filament-wound carbon fibers, and shapes that are nearly spherical. Not only do such tanks cost roughly $3,600 if purchased in large quantities, but their shape does not fit easily into vehicles. [3] Finally, the energy required to compress hydrogen to 10,000 psi equals about 15% of the energy value of the hydrogen in the tank. [4]

Hydrogen condenses into a liquid at –253°C, about 90°C lower than the temperature required for liquid natural gas (LNG). Liquid hydrogen contains nearly double the energy content per volume of hydrogen gas at 10,000 psi, but only 40% of the energy content per volume of LNG. Liquid hydrogen has been used in the space program for decades (the main engine of the space shuttle burns liquid hydrogen and liquid oxygen), and storage systems for liquid hydrogen might be less expensive to mass produce than those for compressed hydrogen gas, on the order of $1200. [5] The process of liquefying hydrogen, however, expends about 30% of the energy contained in the tank. [6] Even greater energy losses occur during storage, when some of the liquid heats up and boils off, and during refueling, when transfers between the service station storage tank and the vehicle requires precooling tubing and valves.

Hydrogen will react with metals such as aluminum, boron, lithium, magnesium, or sodium to form chemical complexes known as hydrides. When subjected to heat or pressure, metal hydrides will release the hydrogen absorbed. Hydrides can achieve hydrogen densities that surpass liquid hydrogen, but they have serious drawbacks. Some of them can be dangerous, exploding upon exposure to moist air. Some are highly toxic to humans. They all require a substantial input of energy, 11% to 26% of the energy contained in the material, for their generation and release. [7] The materials themselves are expensive, on the order of $1600 to $3200 per vehicle. [8] For these reasons, the aerospace industry, although it has conducted extensive research on metal hydrides, has never actually deployed them in a vehicle. Research is also proceeding on systems that absorb hydrogen onto tiny carbon particles (nanotubes or buckyballs). These systems usually operate at temperatures below –200°C. So far, such systems have not reached the efficiencies needed for practical applications.